The crash killed 78 people Wednesday Santiago de Compostela, Spain.
Francisco Jose Garzon was released from a hospital yesterday and detained by police on suspicion of reckless homicide.
He will be questioned by a judge today.
That judge will decide whether to press formal charges.
Investigators believe the train was going more than twice the suggested speed, when it derailed on a curve and crashed.
The judge has until tonight to decide whether to press formal charges against Garzon.
So far, he has refused to make a statement or answer any questions.